As discussed in the related U.S. patent application Ser. No. 08/395,332, in the 1970s and through much of the 1980s, a large percentage of data transfers between two or more separate locations were made using analog modems. An analog modem is a data communication device which modulates data and transmits the modulated data as an analog signal, and also receives modulated data as an analog signal and demodulates the received data. Most analog modems receive and transmit the modulated data as an analog signal over one twisted pair of wires, coupled to the Public Switched Telephone Network ("PSTN") through an analog interface circuit.
In addition, analog leased line connections may also be available for analog data transmission over a dedicated, point-to-point line, which does not require network switching for data transfer and, accordingly, does not involve the PSTN. Analog leased line connections are typically implemented using four wires arranged as two "twisted pairs", although two wire leased line service may be available.
A variety of desirable applications, however, such as downloading data files from and transmitting documents to various networks, may require higher speeds of data transmission than is currently practicable, affordable, or available over most analog telephone (PSTN) lines or analog leased lines. Accordingly, many telecommunication providers have begun to offer and to implement various digital transmission services such as, for example, switched digital services, T1 services, E1 services, and Integrated Services Digital Networks ("ISDN"). Many typical digital connections use two such twisted pairs of wires, one for transmission and the other for reception, coupled to a digital network through a digital interface circuit. A typical T1 connection has a data rate of 1.544 Mbps (megabits per second), which is further time division multiplexed ("TDM")into digital (not analog) transmission channels. The T1 connection comprises 24 channels, with each channel referred to as a DS0 having a data rate of 64,000 bps. ISDN systems typically comprise two 64 Kbps "B" channels (for voice and data transmission) and one 16 Kbps "D" channel (for signalling information and low speed packet data). Specifications and standards for ISDN, T1, and E1 services are described in numerous CCITT Recommendations, such as Recommendation G (for T1, E1), and Recommendation I (for ISDN). Other discussions of analog and digital telecommunications services may be found in a wide variety of references, such as R. Freeman, Reference Manual for Telecommunication Engineers, John Wiley & Sons, 1985.
Digital modems and terminal adapters have typically been used to transfer data over such digital lines. Digital modems typically further encode the modulated analog signal from a modem, using digital encoding schemes such as pulse code modulation ("PCM"), to transfer the analog signal over a digital line. Such digital modems would typically be used when the other (or remote) modem receiving the data is coupled to an analog line and, therefore, cannot receive the purely digital data which could be communicated over a digital line. Terminal adapters are a type of data communication device designed to transmit and receive digitally encoded data directly from a digital network, without intervening modulation of data onto an analog carrier signal or demodulation of an analog carrier signal into digital data.
Separate data communication devices such as analog modems, digital modems, and terminal adapters, are available and known in the art of modern telecommunications. Each such device, however, is typically incompatible for use in another format or mode, as data communications devices have evolved to be either digital or analog, but not both. For example, in the prior art, an analog modem cannot be used to transmit data on a digital network. Correspondingly, in the prior art, neither a digital modem nor a terminal adapter could be used to transmit data on an analog transmission line.
As modern telecommunications transitions to digital formats, existing analog modems ultimately may be rendered obsolete, with a concomitant loss of investment by users in their analog equipment and other technology. In areas where digital network services have become both available and cost-effective to employ, those users that have transferred to digital services may nonetheless need to communicate with analog networks, and vice-versa. For example, many corporations may employ digital networks for internal communications, while continuing to need analog services for external communications, for example, to allow an employee to log in and enter the system from a remote location over the PSTN. In addition, many current analog system users continue to require analog transmission devices, but may want to plan for the future by investing in data communications equipment which will be compatible with both current analog and future digital transmission schemes.
Current solutions to this compatibility problem have been to essentially build separate analog and digital devices, which may then be incorporated into a single housing. For example, Motorola has manufactured various hybrid devices, such as the HMTA 200, which combines a terminal adapter with digital modem functions. Other manufacturers, such as US Robotics, have simply built physically separate "cards" which separately perform these incompatible digital and analog functions and which separately connect to digital or analog interfaces.
Current solutions to the analog and digital incompatibility problem have been inadequate, however, because such current solutions have simply physically combined otherwise separate sets of analog and digital hardware, into one package having both analog and digital interfaces, often with redundant hardware such as microprocessors, RAM and ROM. The need has arisen, therefore, for a single, integrated data communications device which will provide complete analog and digital data transmission services, heretofore provided by separate, independent, and incompatible devices.
This analog and digital incompatibility problem was discussed and addressed in the related U.S. patent application Ser. No. 08/395,332, which disclosed a novel, single, integrated data communications device which may be selectively configured to operate in such analog and digital modes. In the prior art, these disparate communication modes were provided by separate, independent, and incompatible devices.
In addition to this analog and digital incompatibility problem, moreover, many digital system users continue to require back-up analog systems for data communications over the PSTN, in the event of interrupted, disconnected or otherwise unsatisfactory analog leased line or digital network connections. For example, in circumstances in which a T1, an analog leased line, or an ISDN service is employed, in the event of a line failure, the user may require the capability for immediate reconnection and communication over the analog PSTN to avoid, for example, interruption in customer service and communication. Such reconnection and reestablishment of auxiliary communication over the PSTN is often referred to as "dial back-up".
Dial back-up for analog leased line service has typically been available in the prior art. If the analog leased line service is interrupted, disconnected or otherwise lost, the analog modem having dial back-up capability may establish or re-establish a connection over the PSTN for transferring data. Although establishing the connection over the PSTN may cause a delay because of set up time required, the data connection is nonetheless established and data transfer may continue. When the leased line connection is restored, having been previously disconnected, the modem may disconnect from the PSTN and reconnect to the leased line. The Motorola V.3229 modem is one such analog modem configurable for point-to-point data transfers for leased line operation and also configurable for dial back-up over the PSTN.
Because of the analog and digital incompatibility problem, however, current solutions to providing dial back-up for digital services have also been to essentially build separate analog and digital devices, which may or may not then be incorporated into a single housing, with a switching mechanism to transfer from the digital mode to an analog mode in the event of a failure of the digital line. Current solutions to the problem of providing analog dial back-up for digital systems have been inadequate, however, because just as has been the case with solutions to the incompatiblity problem, such current solutions have simply physically combined otherwise separate sets of analog and digital hardware, into one package having both analog and digital interfaces, often with redundant hardware such as microprocessors, RAM and ROM.
Although the arrangement of the local digital modem (or terminal adapter), coupled with the local analog modem for dial back-up, connection cables, and a switching mechanism responsive to a loss of the digital connection, may function to provide reliable back-up service, such a solution requires, at a minimum, two separate data communications devices plus additional switching hardware. The requirement of two devices to implement one data communications function is inefficient and expensive, involving redundant hardware, additional parts, and additional manufacturing costs. In addition, such duplication creates an inefficient use of rack space for housing the data communications devices, and also is an inefficient use of electrical power required to maintain both devices in an operating state. Hence, the need has arisen for a single, integrated data communications device which will provide analog modem dial back-up for the digital network, both for digital modems and for terminal adapters, heretofore provided by separate, independent, and incompatible devices.